Signal processing apparatus

ABSTRACT

This disclosure involves a novel and compact pulse compression filter and the like, employing elastic shear-wave, spin-wave, longitudinal-wave transduction by means of a time-varying magnetic bias field, providing increased power handling capability and greater compression ratios than present-day filters.

K R 3 a 6 6: 8 s 5 6 8 U ,t 7 Umted Stat 3 [151 3,668,568 MorgenthalerJune 6, 1972 54] SIGNAL PROCESSING APPARATUS 3,425,002 1/1969 Okamura..333 30 [72] Inventor: Frederic R. Morgenthaler, Winchester, 3,444,4845/1969 Bierig ..333/30 Mass- OTHER PUBLICATIONS Assigneei Associaies,Harvard, Mass- Pulse Compression Using X-Band Magnetoelastic Wave in[22] Filed: Oct. 6, 1969 Yig Rods" by De Vaart, Proceedings of the IEEE,April 1966,

page 1,007 [21] Appl. No.: 870,478

Primary Examiner-Eli Lieberman Assistant Examiner-Saxfield Chatmon, Jr.[52] US. Cl ..333/30, 333/71, 330/4.6, An0mey RineS and Rims 330/57,332/26, BIO/8.3, 310/9.5 [51] Int. Cl. ..H03h 7/30 57 ABSTRACT [58]Field of Search ..333/30, 71, 31; 330/46, 56,

330 57; 32 2 310/83, 9 This disclosure involves a novel and compactpulse compression filter and the like, employing elastic shear-wave,spin- [56] Reerences Cited wave, longitudinal-wave transduction by meansof a timevarying magnetic bias field, providing increased power han-UNITED STATES PATENTS dling capability and greater compression ratiosthan presentday filters. 3,249,882 5/1966 Stern ..330/56 X 3,353,11811/1967 Olson et al ..333/30 19 Claims, 1 Drawing Figure OUTPUT IN PUTPATEMTEDJUH s :972

OUTPUT INPUT ATTORNEYS SIGNAL PROCESSING APPARATUS The present inventionrelates to signal-processing apparatu's, being more particularly, thoughnot exclusively, directed to producing controlled frequency-dispersivetime delay including time compression, expansion, inversion or otherdelay of electric impulses preferably of the type involvingfrequency-modulated impulses, such as so-called chirp pulses and thelike useful in radar and similar applications, as described, forexample, in'the Proceedings of the'IEEE, Vol. 56, No. 3, March, 1968,pages 273-285.

Numerous types of signal-processing filters have been proposed and usedfor such purposes, including magnetostrictive delay lines, as describedin said Proceedings, electric networks and other devices. All suchtechniques, however, have heretofore had serious limitations inpower-handling capability, in achievable pulse compression (.orexpansion) ratios, and in application to wideband systems.

- It is thus to the obviating of these limitations that the presentinvention is primarily directed; it being an object of the invention toprovide a new and improved signal-processing apparatus of this characterthat has vastly increased powerhandling, pulse-compression ratio, andwideband capabilities.

A further object is to provide a new and improved time signal-processingfilter apparatus of more general utility, also, as well asa novel methodor signal processing.

An additional object is to provide new and improved signaldelayapparatus. I 7

Other and further objects will be hereinafter explained and more fullydelineated in the appended claims. In summary, however, from one of itsbroad aspects the invention contemplates an electric-impulsesignal-processing apparatus having, in combination, anelastic-wave-supporting device, input and output transducer meansdisposed at the device for respectively coupling electric impulsesthereto to generate and propagate elastic waves therein and fortransducing such waves into electric output impulses, and means forapplying a time-varying energy field to the device during the couplingof said electric impulses into the device by the input transducer meansin order to cause different frequencies in said waves to be advanced ordelayed in time by different amounts, thereby to introduce timesignal-processing into the output impulses. Preferred details arehereinafter set forth.

The invention will now be described with reference to the accompanyingdrawing, the single FIGURE of which is a combined longitudinal sectionand schematic circuit diagram of a preferred embodiment. v

It has been determined that microwave elastic waves (phonons)v can. beconverted into spin waves (magnons), and vice versa, by means of aspatially uniform but time-varying magnetic bias field in a singlecrystal yttrium iron garnet (YIG), as described in my article entitled,Phase-Velocity- Modulated Magnetoelastic Waves" appearing in theJournal of Applied Physics, Vol. 37, No. 8, July 1966, pages 3,326-7.Experiments have verified the theoretical prediction that suchconversion occurs at constant wave number and momentum but with variablefrequency, energy and power flux. Advantage is taken of this phenomenon,when modified in accordance with the present invention, to provide,among other things, a novel pulse compression or delay filter that iscompact, versatile, affords greater compression ratios than conventionalfilter, and is suitable for wideband microwave radar systems and thelike.

For purposes of illustration, the invention will be described inconnection with its application as a chirp impulse compression filter.In its simplest form, the filter consists of a single crystalfem'm'agnetic rod (such as YIG) capable of supporting elastic waves andprovided with piezoelectric shear and iongitudinal elastic wavetransducers deposited respectively on the input and output end regionsof the rod. A coil wound around the crystal is used to produce atransient or time-varying magnetic energy field bias. If the bias is aramp function, turned on after an input electric chirp signal isintroduced or coupled to the rod, it can serve to convert each frequencycomponent of the input shear elastic wave, first to a magnetic spin waveand then to a longitudinal elastic wave of relatively fast. velocitycompared to the shear wave and of higher frequency that will couple tothe output transducer. Because high-frequency components spend moretransit time in the rod, proper choice of bias ramp will produce amatched filter output with noise factor improvement. In addition, andbecause of the frequency translation of the output impulse signal, thepulse compression factor is increased by a factor that is the ratio ofthe longitudinal to shear wave velocities V,/V, normally a factor ofabout 2.

Such a filter is flexible because the bias current ramp can be alteredto match a variety of chirp input impulses; and it is compact becausefor input pulses of microsecond duration, the crystal may be about 1centimeter long. Such a filter is potentially wide band (within thelimits of the transducers), moreover, because the linearity of thefilter is as good as that of the bias ramp function. In principle,indeed, microsecond to nanosecond compression is possible. The peak biasenergy required to filterone such pulse, furthermore, is typically a fewmillijoules.

Referring to the drawing, a preferred version of such a filter is shownin section comprising a sphere of polycrystalline YIG 1, for example,diametrically cored and provided with a high quality single-crystalcylindrical rod 1', as of YIG, disposed within the core. Shear-wave (ofthe appropriate polarization) and longitudinal-wave electric-to-elasticwave transducers (such as CdS or ZnO) are deposited on the parallel andoptically flat end faces at 2 and 4, respectively. A solenoid coil 3 iswound about the sphere 1 such that the number of turns per unit lengthas projected along the sphere axis is a constant. The entire structureis magnetized to saturation either by a permanent magnet or anelectromagnet. Current is passed through the coil 3 from a rampgenerator 5, the waveform being shown as the I(t) vs. t graph, producinga substantially uniform spatial magnetic field inside of the spherewhich adds a time-varying component to the bias field provided by magnetN-S. The chirped input pulse is applied to the shear-wave inputtransducer 2, as schematically illustrated by the arrow INPUT, and,during such application, the time-'vary ing ramp-controlled magneticenergy field produced by the solenoid coil 3 acts upon the system.

In operation, consider a frequency component to entering the YIG rod 1via the input shear-wave transducer 2 from the chirp impulse at, say,time it. At a later time t,, the current ramp from generator 5 will haveproduced a sufficient magnetic energy field in the solenoid 3 to causeconversion of the shear elastic wave to a spin wave. The componenttravels a distance V, (t,t) as a shear wave, and then stops (orpropagates at a finite small velocity) for a further time interval(t,t,) until longitudinal elastic-wave conversion occurs at time t,.Finally, the component travels a distance V, (T-t,) as a longitudinalelastic wave until exiting at the output transducer 4 at time T as anoutput signal-processed electric impulse, labelled OUTPUT.

For wave propagation parallel to the internal field (which, in turn, isparallel to the rod axis), the dispersion relation ((0 vs k curve) hasthe form of a first rising curve, followed by a horizontal plateau, asdisclosed in my said article, and then by a continuing or second risingcurve. The plateau region can be moved up and down by varying themagnetic field at 5-3, such that, in effect, the spin waves serve as amagnon elevator," elevating the frequency w from the first rising curveto the second curve where the frequency is thus increased, (or, on theremoval of the ramp, causing the magnon elevator" to drop back again orreverse). The final frequency that emerges at 4, when'the ramp hascaused the magnon elevator thus to rise, is thus higher than the inputfrequency at 2 by the factor V,/V,, where, as before stated, V, and V,are respectively the longitudinal and shear-wave velocities. Since suchhigher frequency is delayed longer in the device, compression iseffected by this frequency-changing (or velocity-changing) time signalprocessing. Power gain also results from this elevating or conversionaction. The power handling capability, indeed,

is inherently about 30 db greater than prior art magnetoelastic devices.The time signal-processing is effected, in accordance with theinvention, by both the conversion between the types of elastic waves(through the spin-wave conversion) and by the differences in velocitytherebetween.

Considering a one centimeter long YIG ferrimagnet magnetized in a "100plane at 22.5 to the l axis or direction, a microsecond chirp pulse withto variation from 21rXl0to 31r l0 rad./sec., a relatively fastlongitudinal wave velocity V, of 7.2 l0"' cm./sec. directed along saiddirection, a slower shear-wave velocity of 3.9 l0" cm./sec., 725 ampereturns in the solenoid coil 3, a ramp duration of LSXlO' sec. for eachsuccessive input impulse, a magnetic field of the order of 400 oe./sec.,and a peak energy delivered to the coil 3 of 1 .3X joules, the followingresults are producible.

The critical time gradients for longitudinal wave conversion will rangelinearly with frequency from about 157 oe./p.sec. at 1.85 Gl-iz to 235oe./p.sec. at 2.77 Gl-lz. The corresponding values of the conversionefficiencies range from 0.324 (-4.89 db) at 1.85 GHz, to 0.444 (-3.53db) at 2.77 Gl-lz. The average conversion loss for the signal pulse isapproximately 4 db. In addition, there is a power gain of (V,/V,) 3.4(+5.3 db), due to the frequency translation and change in groupvelocity. For this operation, the shear elastic wave transducer 2 shouldbe flat over the frequency range l-l.5 GHz; and the longitudinaltransducer 4, over the frequency range l.85 2.77'GHz.

It has been found that at substantially the above-mentioned magnetizingangle of 22.5 to the 100 axis in the 100 plane, maximum interaction orsplitting between the longitudinal and spin wave occurs in the YIGcrystal. Up to about 26 percent improvement in such coupling andconversion has been determined as possible, moreover, for magnetizingand propagating the wave at an angle of substantially 25.52 to the 100"axis or direction in the l 10" plane.

In summary, therefore, a novel and compact pulse compression filteremploying elastic shear wave-spin wave/elastic longitudinal wavetransduction by means of a time-varying magnetic field bias has beenproduced. The linearity of the filter is as good as the current rampfunction can be made; and the bandwidth as wide as that of the input andoutput transducers. Sub-microsecond to microsecond input pulses can beaccommodated. For 500 MHz. chirp bandwidths, compression into thenanosecond range is attainable. Compression is increased by the factorV,/V, over conventional filters. Another outstanding advantage is theease and rapidity with which the filter may be matched to various inputpulses by merely altering the bias waveform. The peak bias energyrequired to filter one pulse is typically a few millijoules; and whileother ferrimagnetic and similar devices may be used, the single crystalYIG appears to be a most suitable device for these purposes. While thespatial gradient of the magnetic field has been described as uniform,moreover, it may also be varied or nonuniform along the crystal device;and other fields besides magnetic fields may, in certain instances, beemployed to create the non-uniform time-varying conversion process.

Further modifications will also occur to those skilled in this art, andall such are considered to fall within the spirit and scope of theinvention as defined in the appended claims.

What is claimed is:

1. An electric-impulse signal-processing apparatus having, incombination, an elastic-wave-supporting device, input and outputtransducer means disposed at the device for respectively couplingelectric impulses thereto to generate and propagate elastic wavestherein and for transducing such waves into electric output impulses,and means for applying a transient time-varying energy field to thedevice during the coupling of said electric impulses into the device bythe input transducer means, and which varies substantially during thetransit of an impulse in said device, in order to cause differentfrequencies in said waves to be advanced or delayed in time by difierentamounts, thereby to introduce time signal-processing into the outputimpulses.

3. An electric-impulse signal-processing apparatus as claimed in claim 2and in which means is provided for converting said spin waves to elasticwaves for transducing into said electric output impulses.

4. An electric-impulse signal-processing apparatus as claimed in claim 2and in which the time-varying field is applied and removed to cause themagnon elevator action to reverse.

5. An electric-impulse signal-processing apparatus having, incombination, an elastic-wave-supporting device capable of supportingrelatively slow and fast elastic waves, input and output transducermeans disposed at the device for respectively coupling electric impulsesthereto in order to generate and propagate therein one of the saidelastic waves and for transducing the other of said elastic waves intoelectric output impulses, and means for applying a transienttime-varying energy field to the device during the coupling of saidelectric impulses into the device by the input transducer means, andwhich varies substantially during the transit of an impulse in saiddevice, in order to cause the said one elastic waves propagating in saiddevice to become converted into the said other elastic waves, thereby tointroduce time signal-processing into the output impulses caused both bythe conversion from the said one to the said other elastic waves and bythe difference in velocity therebetween.

6. An apparatus as claimed in claim 5 and in which said device comprisesmagnetic crystal means and said time-varying energy field is produced bytime-varying magnetic fieldproducing means.

7. An apparatus as claimed in claim 6 and in which said crystal meanscomprises a YIG crystal and said transducer means comprise shear andlongitudinal elastic-wave transducers.

8. An apparatus as claimed in claim 7 and in which the said YIG crystalis a single rod disposed within an outer polycrystal YIG structure aboutwhich the magnetic field-producing means is disposed.

9. An apparatus as claimed in claim 8 and in which the outer YIGstructure is substantially spherical and the said YIG crystal isdisposed substantially diametrically therein.

10. An apparatus as claimed in claim 5 and in which the said electricimpulses comprise a frequency-modulated pulse, the said one and otherelastic waves are respectively the said slow and fast waves, and thesaid signal processing results in compression of the input electricimpulses.

11. An apparatus as claimed in claim 6 and in which the said magneticfield is oriented relative to said crystal means in one of asubstantially 22.5 angle to the axis in the 100 plane and asubstantially 25.52 angle to the 100" axis in the plane.

12. Apparatus as claimed in claim 2 and in which means is provided forspatially varying field along the device.

13. An electric-impulse signal-processing apparatus comprising amagnetic crystal including YIG, and a magneticfield-producing meansoriented to set up its field at one of a substantially 22.5 and a 25.52angle with respect to the 100 axis in the respective 100 and 1 l0planes.

14. Apparatus as claimed in claim 13 and in which means is provided forpropagating elastic waves in said crystal substantially along said 100axis.

15. A method of changing the transit time of relatively low and highfrequency components of elastic impulses, that comprises, propagatingsuch impulses in an elastic-wave-supporting medium, and, during thepropagating, subjecting the medium to a transient time-varying energyfield apart from said impulses which varies substantially during thetransit of an impulse in said medium.

of elastic wave.

18. A method as claimed in claim 17 and in which the said types arerelatively slow and fast elastic waves.

19. A method as claimed in claim 16 and in which there is furthereffected spatial varying of field along the medium.

1. An electric-impulse signal-processing apparatus having, in combination, an elastic-wave-supporting device, input and output transducer means disposed at the device for respectively coupling electric impulses thereto to generate and propagate elastic waves therein and for transducing such waves into electric output impulses, and means for applying a transient time-varying energy field to the device during the coupling of said electric impulses into the device by the input transducer means, and which varies substantially during the transit of an impulse in said device, in order to cause different frequencies in said waves to be advanced or delayed in time by different amounts, thereby to introduce time signal-processing into the output impulses.
 2. An electric-impulse signal-processing apparatus as claimed in claim 1 and in which said field is magnetic and means is provided for converting said elastic waves to spin waves upon which the time-varying energy field acts in the manner of a magnon elevator.
 3. An electric-impulse signal-processing apparatus as claimed in claim 2 and in which means is provided for converting said spin waves to elastic waves for transducing into said electric output impulses.
 4. An electric-impulse signal-processing apparatus as claimed in claim 2 and in which the time-varying field is applied and removed to cause the magnon elevator action to reverse.
 5. An electric-impulse signal-processing apparatus having, in combination, an elastic-wave-supporting device capable of supporting relatively slow and fast elastic waves, input and output transducer means disposed at the device for respectively coupling electric impulses thereto in order to generate and propagate therein one of the said elastic waves and for transducing the other of said elastic waves into electric output impulses, and means for applying a transient time-varying energy field to the device during the coupling of said electric impulses into the device by the input transducer means, and which varies substantially during the transit of an impulse in said device, in order to cause the said one elastic waves propagating in said device to become converted into the said other elastic waves, thereby to introduce time signal-processing into the output impulses caused both by the conversion from the said one to the said other elastic waves and by the difference in velocity therebetween.
 6. An apparatus as claimed in claim 5 and in which said device comprises magnetic crystal means and said time-varying energy field is produced by time-varying magnetic field-producing means.
 7. An apparatus as claimed in claim 6 and in which said crystal means comprises a YIG crystal and said transducer means comprise shear and longitudinal elastic-wave transducers.
 8. An apparatus as claimed in claim 7 and in which the said YIG crystal is a single rod disposed within an outer polycrystal YIG structure about which the magnetic field-producing means is disposed.
 9. An apparatus as claimed in claim 8 and in which the outer YIG structure is substantially spherical and the said YIG crystal is disposed substantially diametrically therein.
 10. An apparatus as claimed in claim 5 and in which the said electric impulses comprise a frequency-modulated pulse, the said one and other elastic waves are respectively the said slow and fast waves, and the said signal processing results in compression of the input electric impulses.
 11. An apparatus as claimed in claim 6 and in which the said magnetic field is oriented relative to said crystal means in one of a substantially 22.5* angle to the ''''100'''' axis in the ''''100'''' plane and a substantially 25.52* angle to the ''''100'''' axis in the ''''110'''' plane.
 12. Apparatus as claimed in claim 2 and in which means is provided for spatially varying field along the device.
 13. An electric-impulse signal-processing apparatus comprising a magnetic crystal including YIG, and a magnetic-field-producing means oriented to set up its field at one of a substantially 22.5* and a 25.52* angle with respect to the ''''100'''' axis in the respective ''''100'''' and ''''110'''' planes.
 14. Apparatus as claimed in claim 13 and in which means is provided for propagating elastic waves in said crystal substantially along said ''''100'''' axis.
 15. A method of changing the transit time of relatively low and high frequency components of elastic impulses, that comprises, propagating such impulses in an elastic-wave-supporting medium, and, during the propagating, subjecting the medium to a transient time-varying energy field apart from said impulses which varies substantially during the transit of an impulse in said medium.
 16. A method as claimed in claim 15 and in which said field is magnetic and said propagating comprises introducing one type of elastic wave into said medium and converting the same to a spin wave therein upon which the time-varying energy field acts in the manner of a magnon elevator.
 17. A method as claimed in claim 16 and in which the spin wave conversion is followed by conversion into a second type of elastic wave.
 18. A method as claimed in claim 17 and in which the said types are relatively slow and fast elastic waves.
 19. A method as claimed in claim 16 and in which there is further effected spatial varying of field along the medium. 